A progesterone-like factor detected by bioassay in the blood of the toad (Bufo bufo) shortly before induced ovulation

Abstract A progesterone-like factor was detected, by bioassay, in plasma from female “winter” toads injected with a dose of HCG sufficient to induce ovulation. Concentration of this factor, which was indicated to be ovarian in origin, was first sufficient to be detected 4 hr after HCG injection, reached a peak after 8–9 hr and fell again to undetectable levels before ovulation began. It is proposed that the progesterone-like factor is released from the preovulatory ovarian follicle following gonadotropic stimulation, and that the high blood level which has been observed is responsible for stimulating release of jelly from the oviducal glands at the time of ovulation. When spawning season toads were used in a second experiment this effect of HCG was not observed.